<template>
  <div class="list">
    <ul>
      <li class="item"
          v-for="item of letters"
          :key="item"
          :ref="item"
          @touchstart.prevent="handleTouchStart"
          @touchmove="handleTouchMove"
          @touchend="handleTouchEnd"
          @click="handleLetterClick"
      >{{item}}
      </li>
    </ul>
  </div>
</template>

<script>
  export default {
    name: "CityAlphabet",
    props: {
      cities: Object
    },
    data() {
      return {
        touchStatus: false,
        startY: 0,
        timer: null
      }
    },
    updated() {
      this.startY = this.$refs['A'][0].offsetTop;
    },
    computed: {
      letters() {
        const letters = [];
        for (let i in this.cities) {
          letters.push(i);
        }
        return letters;
      }
    },
    methods: {
      handleLetterClick(e) {
        this.$emit('change', e.target.innerText);
      },
      handleTouchStart() {
        this.touchStatus = true;
      },
      handleTouchMove(e) {
        if (this.touchStatus) {
          //如果this.timer已经存在，进行清除
          if (this.timer) {
            clearTimeout(this.timer);
          }
          //重新定义this.timer，让其延时滚动
          this.timer = setTimeout(() => {
            const touchY = e.touches[0].clientY - 79;
            const index = Math.floor((touchY - this.startY) / 20);
            if (index >= 0 && index < this.letters.length) {
              this.$emit('change', this.letters[index]);
            }
          }, 16)
        }
      },
      handleTouchEnd() {
        this.touchStatus = false
      }
    }
  }
</script>

<style scoped lang="stylus">
  @import "~styles/varibles.styl"
  .list
    display: flex
    flex-direction: column
    justify-content: center
    position: absolute
    top: 1.58rem
    right: 0
    bottom: 0
    width: .4rem
    .item
      text-align: center
      line-height: .44rem
      color: $bgColor
</style>
